A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapor precursors. One known technique for depositing a thin film on a substrate is chemical vapor deposition (CVD), which is commonly plasma enhanced (PECVD). In this technique, process gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. Examples of gases supplied to the process chamber to form a thin film include, but are not restricted to: silane and ammonia for the formation of a silicon nitride film; silane, ammonia and nitrous oxide for the formation of a SiON film; TEOS and one of oxygen and ozone for the formation of a silicon oxide film; and AI(CH3)3 and water vapor for the formation of an aluminum oxide film.
After processing, the gases exhausted from a processing chamber must be treated prior to release or storage. Gases exhausted from a process chamber can be treated with high efficiency and at a relatively low cost using a plasma abatement device. In the plasma abatement process, the exhaust gas stream is caused to flow into a thermal atmospheric pressure plasma discharge, which is primarily a source of heat. The plasma causes dissociation of the gas stream into reactive species which can combine with oxygen or hydrogen to produce relatively stable by-products.
The different gases used in the cycle of operation of a processing chamber require different amounts of electrical power to be supplied to the torch for effective treatment. Ordinarily, the voltage of the source of electrical energy remains generally constant and change in power is controlled by controlling the supplied current. A typical DC plasma torch has a restricted power range between a lower limit which is defined by the minimum current at which a discharge may be sustained by the torch without quenching and a higher limit which is defined by the maximum current rating of the power supply used or the maximum current which does not result in thermal damage to the torch electrodes.
However, the voltage-current characteristics of a DC plasma torch shows a decrease in voltage with a current increase and vice versa. Thus, even large changes in the torch current result in relatively smaller variations in the power. For example, changing the operating current from 20 A to 40 A (i.e. a 100% increase) increases the torch power by as little as about 60%.